It is known in the art that magnetic bearings have a rotating member (or rotor) and a stationary member (or stator) concentrically located with respect to each other and that the magnetic bearing (and its associated control circuitry) typically controls the radial or axial distance between the rotating rotor and the stationary stator. For example, the stator may be located concentrically internal to the rotor or visa versa. As is also known, a magnetic bearing may use adjustable electro-magnetic forces generated by current flowing through coils wrapped around the stator, as controlled by a control circuit, to adjust the distance between the stator and rotor.
In particular, a "radial" magnetic bearing adjusts the radial distance between the concentrically located stator and rotor. As radial forces are exerted on the rotor, the electro-magnetic forces must be adjusted so as to return the gap between the stator and the rotor to a substantially constant equal value around the circumference of the rotor/stator gap.
Distance sensors are typically used to measure the distance of the gap and to provide input signals to the control circuit to adjust the forces by adjusting the current through the coils. The type of distance sensor commonly used is an inductive or eddy current non-contacting sensor having a diameter of about 1/4", e.g., an inductive sensor by Kaman Instrumentation Corporation, Colorado Springs, Colo., Part No. 854078-001.
A typical inductive sensor comprises two inductors (or coils), one (active) coil which is influenced by the presence of a conducting target and a second (balance) coil which provides temperature compensation. Each coil is typically connected in parallel with a capacitor, and the two coil-capacitor pairs make up two legs of a bridge circuit. The other two legs of the bridge circuit comprise resistors. The bridge circuit has a high frequency ac excitation (e.g., 1 Mhz) applied across two legs of the bridge and an output voltage is measured across the other two bridge legs. Magnetic flux lines from the active coil pass across the measurement gap to a conductive target, producing eddy currents in the surface of the target. As the target comes closer to the active coil, the eddy currents become stronger, which changes the impedance of the active coil and causes a bridge unbalance and a corresponding change in output voltage related to target position. This unbalance voltage may be demodulated, low-pass filtered, and/or linearized to produce a dc output voltage proportional to the gap.
In a radial magnetic bearing, one or more inductive sensors may be used to measure the gap between the rotor and the stator. Thus, if the output voltage from the aforementioned bridge circuit is monitored by the aforementioned control circuit, the control circuit can measure the change in gap and adjust the current in the electromagnetic coils to compensate accordingly. More specifically, the active coil is typically located on one member of the magnetic bearing and the opposite member of the bearing acts as the target. Also, if the opposite (or target) member of the bearing is not conductive or has variable or non-uniform magnetic and/or electrical properties, a metallic ring made of an conductive material, e.g., copper, may be placed thereon to enhance distance measuring performance. For example, if the active coil is located on the stator, the metallic ring would be located opposite the coil on the rotor. Such an inductive sensor is described in U.S. Pat. No. 4,816,759, entitled "Inductive Sensor for Detecting Displacement of Adjacent Surfaces", to G. Ames et al, and U.S. Pat. No. 4,160,204, entitled "Non-contact Distance Measurement System", to R. Denny et al.
For magnetic bearing applications, the gap distance sensors are typically placed as close as possible to the magnetic bearing (and, thus, the electro-magnetic coils), so as to allow precise control of the gap between the rotor and the stator. However, one drawback of using an inductive sensor is that the inductive sensor cannot be placed too close, e.g., less than 0.25 inches, to the electro-magnetic coils of the magnetic bearing. If the inductive sensor is placed too close to the magnetic bearing, the inductive effects caused by changes in electro-magnetic flux generated by changing current through the magnetic bearing coils will corrupt the eddy currents sensed by the inductive sensor, thereby producing an erroneous distance measurement. Another disadvantage of an inductive sensor is that the resistivity of the copper ring changes over time and temperature, which changes the eddy currents and introduces errors in the distance measurement. Further, another drawback to using an inductive sensor is the high cost, e.g., $4,000.00 for a set of four sensors around a radial bearing.
Alternatively, a capacitive-type sensor may be used to determine the distance between the rotor and the stator. In that case, a variable capacitor is used to sense gap distance and the resulting capacitance change is converted into an electrical signal indicative of the gap. The most common form of variable capacitor is the parallel plate capacitor with a variable air gap. However, such capacitive sensors may also be adversely affected by electro-magnetic interference (EMI) generated by the electro-magnetic coils of the magnetic bearing. Further, capacitive sensors are adversely affected by parasitic (or stray) capacitances generated by other nearby metallic components.
Thus, it would be desirable to provide a sensor which can be located very close to or within the electro-magnets of a magnetic bearing, which takes up minimum space, which has a low cost, and which is not sensitive to electro-magnetic interference, thereby allowing for a low cost precise adjustment of the gap between the rotor and the stator at the magnetic bearing.